Semiconductor substrates serve as substrates for the manufacture of certain semiconductor packages onto which semiconductor dice or integrated circuit chips are attached during packaging. Conventionally, substrates are made from iron alloys. However, with an increasing demand for higher performance miniaturized packages, more reactive metals, in particular copper alloy substrates are finding increasing applications in semiconductor packages. These substrates are found to be more attractive than iron alloy substrates due to factors such as better heat dissipation, ease of processing and their lower cost. On the other hand, the disadvantage of copper alloy is that it is prone to oxidation (ie. it reacts with oxygen to produce copper oxide) when exposed to oxygen in the air at high temperatures. Such oxidation results in oxygen forming weak bonds with the atoms at the substrate surface, and a layer of brittle and/or poorly adhering oxides. Thus, oxidation introduces reliability problems for semiconductor packages.
The problem of oxidation is particularly acute during wire-bonding in a typical semiconductor packaging process, wherein conductive bonding wires are bonded to contact surfaces on a semiconductor die and a substrate to establish electrical connections therebetween. In a wire-bonding machine, a substrate may typically be introduced onto a heating plate first for pre-bond heating. Thereafter, the substrate is conveyed to a bonding area to perform wire bonding. After wire-bonding, the substrate needs to be conveyed out of the wire-bonding machine. The substrate should typically be pre-heated to a certain temperature before actual wire-bonding is carried out. Such pre-bond heating may be accomplished by placing the substrate on a heating plate during conveyance to the bond-site. Actual wire-bonding is then commonly done by using an ultrasonic transducer to generate mechanical vibration energy with an external pressure force to bind the wire to the die and substrate surfaces. Heat generated during the conveyance or actual wire-bonding processes may oxidize the surface of the substrate if the substrate is not protected from oxygen in the atmosphere, leading to non-stick or unreliability of the bonds made.
An industry practice for protecting substrates from oxidation is to introduce large amounts of a relatively inert gas, usually nitrogen gas, to the substrate. The nitrogen gas will reduce the amount of oxygen in the immediate vicinity of the substrate, and thus prevent oxidation of the substrate. Therefore, it is preferable for the substrate to be channeled through a nitrogen-rich conduit during the above processes.
One way this can be done is to build a so-called heat-tunnel wherein the substrate is conveyed through an enclosed space filled with nitrogen gas, such that exposure of the substrate to the atmosphere is minimized. However, as will be explained in more detail with reference to FIGS. 1 & 2, the conventional heat-tunnel system has its disadvantages. As shown in FIG. 2, the heat-tunnel comprises a work holder system, tunnel cover and a pin indexer, each of which is designed for a particular shape or size of substrate. If a different substrate were to be used, all the parts comprising the heat-tunnel have to be changed. As a result, longer down-time in the production line is incurred, and the changes demand repeated design effort and costs. Skill is needed to reset the system mechanically. Moreover, the use of the indexing pin to move the substrate through the heat-tunnel is inflexible, in that different sizes of pins have to be used for different sizes of corresponding indexing holes on the substrates and a slot in the tunnel cover allowing for pin movement has to be appropriate to cater to the pitching distance of the substrate in question.